1. Field of the Invention
The invention relates to the production of perchloroethylene and trichloroethylene by catalytic oxychlorination of C.sub.2 hydrocarbons or their partially chlorinated derivatives, to catalysts useful in such oxychlorination processes, and to processes for making such catalysts.
2. General Background and Summary of the Prior Art
Perchloroethylene, C.sub.2 Cl.sub.4, and trichloroethylene, C.sub.2 HCl.sub.3, are chlorinated hydrocarbons which have been widely used as solvents in dry cleaning textiles, in degreasing metal parts, in various solvent extraction processes, in compounding rubber cements, and in various other operations. Perchloroethylene is relatively stable, so its use is currently much less severely restricted by anti-pollution regulations than is the use of most other chlorinated solvents. Trichloroethylene is a potentially valuable intermediate for manufacturing chlorofluorocarbon replacements which are less damaging to the atmospheric ozone layer.
It has been common to produce perchloroethylene simultaneously with trichloroethylene by catalytic oxychlorination of ethane, ethylene or a partially chlorinated derivative thereof, i.e., by reacting such a feedstock with chlorine or hydrogen chloride and air or oxygen at a suitable elevated temperature in the presence of a suitable solid catalyst, the catalyst being maintained in the reaction zone either as a fixed bed or, more preferably, as a fluidized bed.
Typically, such oxychlorination catalysts are compositions that comprise a catalytic amount of a metal having a variable valence, notably copper, as well as an alkali metal, notably potassium, supported on a suitable carrier. Carriers used commercially in the past have included highly calcined Fuller's earth (an adsorbent clay) such as "Florex" or, preferably, synthetic activated aluminas. See, for instance, U.S. Pat. No(s). 3,267,162, 3,296,319 and 4,463,200. Fuller's earth is essentially a porous magnesium-aluminum silicate containing small proportions of oxides of iron, calcium, potassium and titanium. By contrast, synthetic activated alumina consists essentially of alumina with virtually no significant impurities or at the most only a very small proportion of silica.
Researchers working on catalytic oxychlorination processes in the prior art have in some instances expressed a preference for the use of low-surface-area alumina as catalyst supports, i.e., for supports having a surface area below 10 m.sup.2 /g or more especially between 2 and 5 m.sup.2 /g (as in U.S. Pat. No. 4,124,534), while in other instances they have expressed a preference for high-surface-area alumina as catalyst supports, i.e., for supports having a surface area of at least 100 m.sup.2 /g (as in U.S. Pat. No. 4,463,200). Low-surface-area supports have been recommended mainly because they were thought to result in higher HCl conversions and lower carbon burning (see, for instance, U.S. Pat. No. 3,427,359 and French Patent No. 1,386,023), whereas high-surface-area supports have been recommended because they were thought to contribute to an increased selectivity of the reaction toward the production of perchloroethylene as the desired product and a reduced formation of undesirable 1,1,2-trichloroethane and unsymmetrical tetrachloroethane, as indicated in U.S. Pat. No. 4,463,200.
However, catalysts comprising a low-surface-area support have been found to be relatively unstable in that such supports possess only a relatively small number of binding sites for retaining the active metal salts in the composition, and the resulting loss of the metal salts from such catalysts has been found to constitute a significant factor in causing corrosion of the metal reactors in which such oxychlorination reactions are generally carried out. In addition, especially when low-surface-area supports such as diatomaceous earth or other silica-aluminas are used, high selectivities to perchloroethylene and trichloroethylene are not obtained at reasonable temperatures.
On the other hand, catalysts comprising a high-surface-area support, especially when used at temperatures near or above 400.degree. C. which are required to obtain good selectivity for the production of perchloroethylene and trichloroethylene, cause substantial oxidation of the feed material with a concomitant formation of unwanted carbon oxides, especially when ethane or ethylene are used as the feed, and in particular under conditions favoring the production of trichloroethylene.